1. Field of the Invention
The present invention relates to an automatic control method and apparatus for controlling an object to be controlled in a closed loop.
2. Description of the Prior Art
It is well known that, in general, a closed loop control is employed for controlling automatically an object to be controlled such as an electrically driven valve or the like. According to common practice, a dead zone is usually provided in the automatically controlling system which includes movable parts such as gears and a drive motor, so as to prevent the control operation from taking place at a higher frequency. In this case, only when the deviation or error between a desired quantity and an actually controlled quantity of an object to be controlled comes out beyond the dead zone, the control operation is performed, whereby the frequency of the control operation can be decreased.
According to the hitherto known control system, however, the control operation is stopped immediately after the controlled quantity falls within the dead zone. Consequently, in the situation where the desired quantity undergoes variations, a correspondingly increased number of control operations will take place. Therefore, the provision of the dead zone which is inherently intended for preventing the frequent control operations will not always bring about the intended action in a satisfactory manner. An attempt to overcome such disadvantage is disclosed in Japanese Patent Publication No. 15607/1966 entitled "Automatic control apparatus". According to this prior art reference, control is made such that when the controlled quantity comes out beyond the dead zone, the size or range of the dead zone is reduced substantially to zero to thereby cause the controlled quantity to coincide with the desired quantity.
However, the control method disclosed in the above-mentioned prisn art reference is disadvantageous in that the size or range of the dead zone has to be altered every time the actually controlled quantity comes out beyond the dead zone. For altering the size or range of the dead zone, there is conceivable an arrangement for adjusting the dead zone setting elements which constitute the dead zone circuit. Alternatively, a pair of the dead zone circuits are provided and interconnected so as to be mutually exchangeable. However, both of these solutions involve a complicated circuit arrangement with a rather degraded reliability in attaining the intended control.